Exploring the Essence of Shimmer

The shimmering iridescence on the petals of the Queen of the Night tulip is not only a delight to gardeners but also of interest to scientists. The deep purple tulip’s multicolored sparkle is an example of what researchers call structural color, that is, an optical effect produced by physical structure rather than chemical pigment. Tiny ridges in the petals of the flower split white light into colors, like a prism, creating the jewel-like effect.

Researchers at McGill University in Montreal have shown how plant cellulose, a biological liquid crystalline material, can self-assemble into wrinkled surfaces to create optical effects such as iridescence and color change. Their work was published in the Journal of Chemical Physics (doi: 10.1063/1.4929337).

The iridescent shimmer of the Queen of the Night tulip is caused by microscopic ridges on its petals that diffract light. Courtesy of S. Vignolini.
“We provide a good foundation based on the well-established liquid crystal models and principles to further explore structural color in nature,” said doctoral candidate Pardis Rofouie. She worked on the project with professors Damiano Pasini and Alejandro Rey, who led the study.

The group expects that insight gained from their research can be used to generate surface patterns and nanoscale actuation systems that can autonomously respond to humidity, leading to the design of optical devices such as humidity sensors.

Cellulose fibers are found in layers within the cell walls of green plants. The fibers in a single layer tend to align in a single direction. The axis of orientation of the fibers in the individual layers can shift, however, forming a twisting pattern, which is called a cholesteric phase because it was first observed in cholesterol molecules. Scientists believe the twisting pattern helps to provide structural strength.

The twisting structure could also produce optical effects such as those seen in the iridescent tulips, the researchers believed.

To explore this idea, they constructed a computational model to study the behavior of cholesteric-phase cellulose. In the model, the axis of twisting runs parallel to the surface of the cellulose. The researchers found that subsurface helices — curves in 3D space — naturally caused the surface to wrinkle, forming microscopic ridges that were spaced apart on the order of microns. They found the pattern of the parallel ridges was similar to the microscopic pattern on the petals of the Queen of the Night tulip. In this process of diffraction grating, the ridges split light into color, creating iridescence.

In this representation of the cholesteric phase, cellulose fibers in a single layer align in a single direction. Traveling through multiple layers, the axis of orientation of the fibers spins in a circle. Courtesy of Abigail Malate.
The group observed that the amount of water stored in the cellulose layers also plays a role in a plant’s optical effects. More water in the layers made them twist less tightly, which in turn made the ridges farther apart. How tightly the cellulose helices twist is called the pitch. A surface with varying pitch, wherein some layers held more water than others, was less iridescent and reflected a longer primary wavelength of light than surfaces with a constant pitch.

“The self-assembly formation of the surface ultrastructure together with the water-induced multiple structural colors suggests a potential mechanism to be exploited in the design of colorimetric humidity sensors,” Rofuie said. “The potential biosensors can respond to different ranges of relative humidity depending on the amplitude and wavelength of the grating structure that can be estimated using our computational liquid crystal model.”

Most of the structural colors in nature originate from optical processes such as thin-film interference, multilayer interference, diffraction grating effects and photonic crystals, she said.

“In the future, we would like to investigate other mechanisms through which nature generates structural colors, especially the multilayer interference that is responsible for iridescent colors observed in Selaginella willdenowii leaves and Pollia condensata fruit.”